1. Field of the Invention
This invention relates generally to a color television receiver, and more particularly is directed to an improved color hue control circuit therefor.
2. Description of the Prior Art
Recently, it has been proposed to provide a color television receiver with a color hue control circuit which causes the color picture tube to display a standard color bar, for example, made up of horizontally successive areas of primary colors, such as, red, green and blue, representing a predetermined standard hue, and further causing the color picture tube to display a reference color bar at a location on the screen adjacent the standard color bar and which is similarly composed of horizontally successive areas of red, green and blue colors which are varied in accordance with manual adjustment of the hue of the color picture otherwise reproduced on the screen. With the foregoing arrangement, when the hue of the color picture reproduced or displayed on the screen corresponds to the predetermined standard hue, the colors of the red, green and blue areas of the reference color bar exactly correspond with the red, green and blue areas, respectively, of the adjacent standard color bar so that the attainment of the standard hue can be easily and accurately ascertained merely by a direct visual comparison of the respective areas of the standard and reference color bars. Similarly, deviation of the hue of the color picture being reproduced on the screen from the predetermined standard hue is easily ascertained by a visual comparison of the respective colored areas of the standard and reference color bars, and the viewer can then manually actuate the hue adjustment knob or control of the television receiver until the respective colored areas of the standard and reference color bars become identical. Prior to the described arrangement providing comparable standard and reference color bars on the screen of the color picture tube, it had been the usual practice to determine the suitablity of the hue of the displayed color picture by observing areas of the reproduced color picture representing human skin, and to manually effect the hue adjustment necessary to provide such areas of the reproduced color picture with life-like skin tones. In the event that the displayed color picture did not include areas representing human skin, considerable difficulty was experienced in achieving suitable adjustment of the hue. On the other hand, when comparable standard and reference color bars are provided on the screen of the color picture tube, as aforesaid, accurate adjustment of the hue of the reproduced color picture can be easily effected even when the reproduced color picture does not contain any areas representing human skin.
As is well known, in a conventional NTSC composite color television signal, monochrome information, that is, the brightness information in respect to the video picture, is transmitted by a luminance component, and color information is transmitted by a chrominance component which is constituted by a plurality of color signals amplitude modulated on different phases of a subcarrier. The composite color television signal also includes periodic horizontal and vertical synchronizing signals and color burst signals which are used, in the television receiver, to control a local oscillator so that the local oscillation signal produced by the latter will have its frequency and phase synchronized with the frequency and phase of the color burst signal. Such local oscillation signal is then employed in a demodulator, for example, constituted by synchronous detectors, for demodulating or detecting the color signals of the received chrominance component which are suitably applied to the color picture tube for establishing the color values of the reproduced picture. If the composite color television signal, as received, exactly corresponds to the originally produced color television signal, then the above described color hue control circuit will provide the reproduced color picture with the predetermined standard hue whenever the respective areas of the standard and reference color bars are seen to be of exactly the same colors. However, a transmitted composite color television signal may be subjected to various phase and/or amplitude distortions either at the transmitting site or along the transmitting path. Although the purpose of the color burst signal is to permit the local oscillator of the color television receiver to be controlled so that its output will match or track such distortions, the color burst signal and the color-modulated subcarrier are provided at different levels of the transmitted signal and, thus, may be subjected to different distortions. Consequently, even though the local oscillation signal is phase synchronized with the color burst signal, there may nevertheless be a phase distortion of the color-modulated subcarrier in respect to the local oscillation signal used for demodulating the color signals with the result that errors may appear in the hue of the color picture displayed on the screen even though a visual comparison of the previously mentioned standard and reference color bars would indicate that the displayed or reproduced color picture has the predetermined standard hue. In such case, the viewer must again adjust the color hue with reference to the reproduced color picture, and preferably with reference to areas of such picture representing human skin, rather than by a comparison of the standard and reference color bars. Furthermore, since the phase distortion of the color subcarrier may vary with time, it becomes necessary to repeatedly adjust the hue of the reproduced color picture.
Various proposals have been offered for using a further reference included in the transmitted composite color television signal to minimize the effect of those distortions in the color subcarrier which may not be fully corrected by synchronizing the local oscillator of the color television receiver with the received burst signal. Such further reference signal is known as the vertical interval reference signal, or VIR signal, and includes a chrominance reference portion or bar equal in frequency and phase to the burst signal, and being transmitted during a selected portion, for example, the 19th or 20th horizontal line interval, of the vertical retrace interval. The chrominance reference portion or bar of the VIR signal differs from the burst signal only in that it exists for a longer duration and is superimposed on a level approximating that of the color subcarrier so as to be subject to phase distortions equivalent to those of the color subcarrier.
In one existing circuit arrangement for controlling color hue on the basis of the VIR signal, for example, as disclosed in U.S. Pat. No. 3,755,617, the local oscillator of the television receiver is controlled so that its output or local oscillation signal is synchronized with the received burst signal, the chrominance reference portion or bar of the received VIR signal is phase detected by means of the burst signal to produce a compensation signal representing the phase difference between the chrominance reference portion of the VIR signal and the burst signal, and the phase of the local oscillation signal is shifted or compensated in dependence on the compensation signal so that, when the local oscillation signal is thereafter used for demodulating the color signals, the resulting reproduced or displayed color picture will exhibit the optimum or standard color characteristics. Although the foregoing arrangement is effective to automatically maintain an optimum or standard hue of the reproduced color picture, a television viewer may have a subjective preference for a color picture with a hue other than the optimum or standard hue, and no provision has been made for adjustment of the color hue from the optimum or standard hue.
In another existing circuit arrangement utilizing the transmitted VIR signal, for example, as disclosed in U.S. Pat. No. 3,780,218, a differential amplifier is made operative during the occurrence of the chrominance reference portion or bar of the VIR signal and the R-Y output of the color demodulator is applied to one input of the differential amplifier, while a fixedly predetermined reference voltage is applied to another input of the differential amplifier so that a phase distortion of the chrominance reference portion, and hence of the color subcarrier, will be evidence by a difference between the voltages at two outputs of the differential amplifier. In one embodiment of the arrangement being described, a meter is connected between the outputs of the differential amplifier so as to be deflected from its zero or null position whenever the hue of the reproduced or displayed color picture deviates from the optimum or standard hue, and a manually actuable hue control is associated with the reference or local oscillator so that the phase of the output of the latter can be adjusted for returning the meter indication to its null or zero postion indicating attainment of the optimum or standard color hue. In such embodiment of the known circuit arrangement, the VIR signal is only employed as a reference for indicating, by way of the meter, when the optimum or standard hue has been attained in response to manual actuation of the hue control, and no provision is made for automatic maintenance of the optimum or standard hue.
In another embodiment of the circuit arrangement disclosed in U.S. Pat. No. 3,780,218, the outputs of the differential amplifier are applied to a phase control or variable phase shifter acting on the output of the reference or local oscillator so as to automatically maintain the optimum or standard hue of the displayed color picture. With such automatic circuit arrangement, there is no possibility of the viewer establishing a subjectively preferred hue differing from the optimum or standard hue.